This means that the bell pushbutton is absolutely dustproof and waterproof.
Opener and turnkey
There are different ways of working buttons. The most common are normally closed and normally open.
An opener interrupts the circuit when actuated. Opener are also called normally closed contacts.
Normally the contacts are marked with the digits 1 and 2 (input and output) or NC (Normally Close).
A turnkey closes the contact when actuated. Turnkey are also called normally open contacts.
In this case, the digits 3 and 4 (input and output) or NO (Normally Open) are used for identification purposes.
NO contacts are often used as bell buttons.